There is widespread interest in ophthalmology to change the refractive state of the eye itself. The human eye has a strength of 60 diopters with the cornea supplying on the average two-thirds, or 40 diopters, of strength. The remaining power of 20 diopters resides in the lens of the eye. Thus, the living cornea itself contributes significantly to the focusing of light by the eye.
Consequently, it is not surprising that attempts to change the refractive state of the eye, that is to eliminate nearsightedness, farsightedness, or astigmatism, have concentrated on changing the shape of the cornea. This is the most accessible portion of the eye and the one with the most refractive power. In this regard, 99% of all refractive error lies between zero and .+-.20 diopters, and 95% between zero and .+-. seven diopters.
Current procedures do exist that change the shape of the cornea. For example, radial keratometry involves making radial cuts in the cornea to flatten or reduce corneal power. Other procedures involve removing part of the cornea, lathing it to a new shape, and suturing it back in place.
In addition, the use of the excimer laser has been proposed to change the shape of the cornea either by direct or indirect lathing of the cornea or by using the laser beam to make radial incisions. The ultraviolet light of the excimer laser is completely absorbed by biological tissue such as the cornea, and upon such absorption photoablation takes place. This is the distintegration of tissue without deleterious thermal effects. Using this laser energy, it is possible thereby to remove in tact corneal tissue for the human eye layer by layer and thereby change its refractive power.
However, to date these various procedures have had numerous shortcomings. For example, the radial keratometry procedure is not exact and often the change in curvature of the cornea is not precisely accomplished. Additional side effects from this procedure are also recorded wherein patients have an oversensitivity to natural light.
Moreover, when using excimer lasers, it is very difficult to precisely control the laser energy and if this is not done, the ablated corneal surface will be changed irregularly and vision worsened.
Examples of publications dealing with attenuation of lasers, photoablation of corneas and other treatments thereof are as follows: J. Taboada et al, "Response of the Corneal Epithelium to KrF Excimer Laser Pulses", Health Physics, Vol. 40, May 1981, pp. 677-683; L. J. Girard, "Advanced Techniques in Ophthalmic Microsurgery", Corneal Surgery, Vol. 2, 1981, Chapters 3-6; D. F. Muller et al, "Studies of Organic Molecules as Saturable Absorbers at 193 nm", IEEE Journal of Quantum Electronics, Vol. QE-18, No. 11, November 1982, pp. 1865-1870; K. Bennett et al, "Variable Laser Attenuators--Old and New", Laser Focus, April 1983; S. L. Trokel et al, "Excimer Laser Surgery of the Cornea", Am. J. of Ophthalmology, Vol. 96, December 1983, pp. 710-715; A. M. Cotliar et al, "Excimer Laser Radial Keratotomy", Ophthalmology, Vol. 92, No. 2, February 1985, pp. 206- 208; and C. A. Puliafito et al, "Excimer Laser Ablation of the Cornea and Lens", Ophthalmology, Vol. 92, No. 6, June 1985, pp. 741-748.
Examples of U.S. patents relating to laser surgery include U.S. Pat. Nos. 4,461,294 to Baron and 4,469,098 to Davi, the disclosures of which are hereby incorporated by reference.